1. Field of the Invention
The present invention relates to an object, a method, a manufacturing method, a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor device, a display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof. Specifically, the present invention relates to an array controller and a storage system.
2. Description of the Related Art
In recent years, storage systems including a plurality of mass storage devices such as hard disks (hard disk drives (HDDs)) have been widely used. In general, a storage system includes a plurality of storage devices and a controller for controlling access to these storage devices (array controller or disk array controller).
Examples of such a storage system including a mass storage device are a JBOD (just a bunch of disks) system and an RAID (redundant array of inexpensive disks or redundant array of independent disks) system.
A storage system, which is required to have high reliability, is provided with a cache system for securing data coherency in the case of a system stop due to power failure or the like (see Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4).
Securement of data coherency and protection of data in the case of a system stop due to power failure or the like are important factors in enhancing the reliability and availability (capability of a system to operate continuously) of storage. For example, when a system is stopped, data whose writing is not completed is stored in (backed up to) a cache system which operates at high speed, so that the data stored in the cache system can be read out when the system is recovered (restarted); thus, incoherency of data in the case of a system stop can be prevented.
A specific example of such a cache system is a system in which a volatile memory, such as a dynamic random access memory (DRAM), is used as a cache memory and a power storage device for a cache memory is used. A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is accumulated in a capacitor. Since data stored in a DRAM is lost when power supply is stopped, a power storage device for a cache memory is needed for securing power supply in the case of power failure.
Instead of using a DRAM and a power storage device, a nonvolatile memory, such as a flash memory, can be used for a cache system for securement of data coherency and protection of data in the case of power failure.
A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding charge in the floating gate. Therefore, a flash memory has advantages in that the data holding period is extremely long (semi-permanent) and refresh operation which is necessary to a volatile memory is not needed.